1. Field of the Invention
The present invention relates to a sensor, an apparatus incorporating the sensor and a method of fabricating the sensor to measure chloride ions; and in particular to measuring chloride ions in difficult conditions, such as varying wet and dry conditions in structural concrete with high pH for a duration of years.
2. Description of the Related Art
The United States is replacing aging infrastructure (such as buildings, bridges and piers) and simultaneously developing the tools and techniques to monitor new infrastructure as it ages. In particular, the US Department of Transportation and other organizations are developing approaches to monitor these structures in order to manage their maintenance and rehabilitation.
Currently, trained personnel base most infrastructure monitoring on individual sensor measurements or periodic visual inspection. These measurements are not distributed throughout the structure being monitored; they are used only as sparse samples to assess general health of the structure. The structure is not thoroughly sampled, mostly due to economic limitations. The cost of these measurements is high because of installation and monitoring requirements. This approach doesn't detect degradation until it has already reached an advanced state. When advanced degradation does occur, the corrective actions are more expensive than if the progress of degradation had been detected at a less advanced stage. Additional costs are incurred because the resources consumed to repair a highly degraded structure diminish scheduled repairs and maintenance for other structures. In addition, costs are imposed on the users of those structures, as their travel or utilization is diverted for extended periods of time.
In a recent approach, a robust, reliable, long-lasting and cost-effective instrumentation package is embedded in the infrastructure to measure the parameters of the physical processes that cause the degradation. The goal is to monitor the physical parameters so that a degrading environment can be detected and maintenance or remediation implemented early—before advanced degradation actually occurs. By focusing on measuring the physical processes at a large number of locations in the structure being monitored, the environmental states associated with the processes can be detected, and even relatively localized areas of degradation can be determined. When monitored at different times, these can be used to indicate the evolution of the infrastructure environment with time. Detecting the onset and evolution of the processes causing the degradation enables corrective action to be scheduled and implemented before significant degradation occurs. Such early remedial action is less expensive and disruptive than the more costly and time-consuming repair or replacement required after advanced degradation.
For example, in bridge decks, corrosion of rebar is the primary cause of degradation. Corrosion is accelerated as salt water is transported through the concrete. Salt water includes constituents such as sodium chloride, NaCl. Salt water originates in thaw water on structures that have been de-iced with salt and on seaboard structures subject to salt spray from the ocean or other body of salt water. Empirical relationships for corrosion rate based on concrete electrical conductivity and other environmental parameters including chloride ion concentration have been developed. These conductivity relationships are interpreted to represent a host of transport properties that result in corrosion and the relationships have considerable scatter. Thus, conductivity has value as a gross indicator of the probability of corrosion but only in the context of field measurements taken in particular environments. In particular, it has been found that corrosion rate is independent of conductivity without the presence of chloride ion. Since the presence of chloride ion plays an essential role in rebar corrosion, a measure of chloride ion concentration provides a strong indication of the corrosive environment of a structure. For such cases, the recent approach is based on set of embedded sensors including temperature, conductivity and chloride ion, among others.
A well-known chloride ion sensor is based on silver (Ag) electrodes with silver—chloride (AgCl) coating, called Ag/AgCl wire electrodes. The sensor functions due to the sparingly soluble nature of AgCl.
While suitable for many purposes, a sensor based on Ag/AgCl wire electrodes suffers from some disadvantages. One disadvantage is that the longevity of the Ag/AgCl wire electrodes is limited to about three months of operation when imbedded in concrete, well short of the decades of life desirable for monitoring bridge decks. The functionality of the silver electrodes is of limited duration due to the high pH (alkalinity) in concrete. Analysis of the chemical processes associated with Ag/AgCl electrodes in concrete indicates that the likely failure mode is erosion of the AgCl to form silver hydroxide (AgOH). At high values of pH, large concentrations of hydroxide ion (OH−) exist. Hydroxide also forms a sparingly soluble compound with silver, AgOH. The thin coatings of AgCl are likely to degrade with the repeated attack of large amounts of hydroxide.
Another disadvantage of the sensor based on Ag/AgCl wire electrodes is that it is insensitive to low chloride ion concentrations. A further disadvantage is that the Ag/AgCl wire electrodes must be wet, in order that environmental chloride ions are mobile between the electrodes at the time of the measurement. This is a disadvantage for making measurements in concrete that is dry. Concrete in infrastructures is often dry at times when it is convenient to make measurements, and concrete at several inches from the surface may be dry even after many rain events. A further disadvantage is that the electrodes are formed as wires that extend into the medium. Such extensions are easily damaged during a process to embed the sensors in the medium. Further disadvantages occur under freezing conditions. The Ag/AgCl electrode requires a reference electrode that usually has some sort of electrolyte inside. Upon freezing this electrolyte precipitates its ionic constituents; and those constituents may not re-dissolve well. Also, as with no moisture, frozen moisture does not allow mobile ions that provide a conductive path to support a voltage measurement between electrodes.
Another approach for measuring chloride ions is described in an article by Shaojun Dong, Zhisheng Sun and Ziling Lu, “Chloride Chemical Sensor Based on an Organic Conducting Polypyrrole Polymer”, ANALYST, vol. 113, October 1988, (hereinafter Dong) the entire contents of which are hereby incorporated by reference as if fully set forth herein. This approach determines ion concentration based on potential difference between a reference electrode and an electrode coated with a particular polymer selective for the chloride ion. Dong demonstrated that a chloride-ion-selective electrode could be made by electro-polymerization of pyrrole with a supporting electrolyte solution of 0.1 M LiCl. The reason that this happens, though not explained in the article, is that the approach effectively employs chloride imprinting of cross-linked polymers. Polypyrrole is an extensively cross-linked polymer. Electro-polymerization of polypyrrole results in over oxidation that yields an average of one positive charge distributed over four pyrrole units. This positive charge is compensated in this case by chloride anions. By forming the polymer in this manner, the polymer has essentially become molecularly imprinted for chloride ion. Application of a negative potential to the chloride polymer will cause it to expel chloride ions. Use of the polymer-coated electrode in an electrochemical cell allowed the selective measurement of chloride ion, proving that some form of imprinting had occurred.
While suitable for many purposes, the approach of Dong also suffers some disadvantages. One disadvantage is that the reference electrode and polymer-coated electrode must be wet. As stated above, this is a disadvantage for making measurements in concrete that is often dry. A second disadvantage is that the sensor is not designed for the high alkaline levels found in concrete (pH greater than 10). As stated in Dong, the sensor response is relatively constant in acidic to neutral solutions (pH 2.5 to 7) but tails off at higher pH levels. A third disadvantage is that the electrodes are formed as wires that extend into the medium and are easily damaged during a process to embed the sensors in the medium.
Based on the foregoing description, there is a clear need for techniques for measuring chloride ion concentration that do not suffer the disadvantages of the prior art chloride ion sensors. For example, there is a need for chloride sensors that measure chloride ion concentrations in a wet or dry medium. There is also a need for chloride sensors that measure chloride ion concentrations in highly alkaline conditions.
In particular, there is a need for long-lived chloride sensors that measure chloride ion concentrations significant for determining multiple degrees of rebar corrosion in reinforced concrete whether the concrete is wet or dry at the time of the measurement.